In drilling bore holes in earthen formations by the rotary method, rock bits fitted with one, two or three rolling cutters are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface, or the bit is rotated by downhole motors or turbines. The cutters mounted on the bit roll and slide upon the bottom of the bore hole as the bit is rotated, thereby engaging and disengaging the formation material to be removed. The roller cutters are provided with cutting elements that are forced to penetrate and gouge the bottom of the borehole by weight of the drillstring. The cuttings from the bottom and sidewalls of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow drillstring.
One type of cutting element in widespread use is a tungsten carbide insert which is interference pressed into an aperture in the cutter body. Tungsten carbide is metal which is harder than the steel body of the cutter and has a cylindrical portion and a cutting tip portion. The cutting tip portion is formed in various configurations, such as chisel, hemispherical or conical, depending upon the type of formation to be drilled. Some of the inserts have very aggressive cutting structure designs and carbide grades that allow the bits to drill in both soft and medium formations with the same bit.
Although very successful, several areas in the world have relatively soft non-abrasive formations which can cause severe frictional heat cracks to the outer ends of the inserts which rub on the borehole wall. Premature failure of the heel row inserts occurs when harder formations are encountered later in the run.
Another type of rolling cutter earth-boring bit is commonly known as a "steel-tooth" or "milled-tooth" bit. Typically these bits are for penetration into relatively soft geological formations of the earth. The strength and fracture-toughness of the steel teeth permits the use of relatively long teeth, which enables the aggressive gouging and scraping actions that are advantageous for rapid penetration of soft formations with low compressive strengths.
However, it is rare that geological formations consist entirely of soft material with low compressive strength. Often, there are streaks of hard, abrasive materials that a steel-tooth bit should penetrate economically without damage to the bit. Although steel teeth possess good strength, abrasion resistance is inadequate to permit continued rapid penetration of hard or abrasive streaks. Consequently, it has been common in the arts since at least the 1930s to provide a layer of wear-resistance metallurgical material called "hardfacing" over those portions of the teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast or macro-crystalline tungsten carbide, dispersed in a steel matrix. Such hardfacing materials are applied by welding a metallic matrix to the surface to be hardfaced and applying the hard particles to the matrix to form a uniform dispersion of hard particles in the matrix.
Unlike a tungsten carbide insert bit, teeth of a steel-tooth bit are not susceptible to stress cracking due to excessive heat. A steel-tooth bit would be able to drill the relatively soft non-abrasive formations mentioned above which cause stress cracking on heel rows of insert bits. However, because of the hardness and thickness of adjacent formations, a steel-tooth bit would wear too quickly, thus is not a preferred choice in those areas.